Several types of self limiting electrical heating elements are known from, e.g., German patent No. 2,543,314 and the corresponding U.S. Pat. Nos. 4,177,376, 4,330,703, 4,543,474, and 4,654,511.
Further, U.S. Pat. No. 5,057,674 describes such an element comprising two outer semiconductive layers allegedly having a zero temperature coefficient (“ZTC”) separated from one another by a continuous positive temperature coefficient (“PTC”) layer and energized by two parallel electrodes, the first one being in contact with one end of one of the ZTC layers and the second parallel electrode being in contact with the other ZTC layer at its end furthest removed from the first electrode.
According to U.S. Pat. No. 5,057,674 the components of the layered structure are such that at room temperature, the resistance in the PTC layer between the ZTC layers is very much less than the resistance in the combined ZTC layers, which in turn is very much less than the resistance in the PTC layer between the electrodes. Further, at control temperature the resistance in the PTC layer between the parallel ZTC layers should be equal to the resistance in the parallel ZTC layers, the geometry being such that at the control temperature where the resistances of the two components are equal, the heat generated per time and unit area (the power densities) are also essentially equal.
The PTC layer at room temperature acts as a short circuit between the parallel ZTC layers. The resistance between the electrodes in the PTC layer is very high when a voltage is at first applied and the ZTC layers alone develop heat, this is a result of the geometry. However, as the temperature rises the resistivity in the PTC layer increases until it is equal to that of the combined ZTC layers. Slightly above this temperature the two ZTC layers act as electrodes and heat is generated uniformly throughout the system, and any further rise in temperature anywhere in the area of the ZTC layers effectively reduces or shuts off the current. In this way the PTC component acts almost only as a control, and the ZTC components perform as the active heating elements.
Also according to this patent the polymer matrix is essentially crystalline, the given example being polyethylene (PE) and ethylene vinyl acetate (EVA).
A problem with both this heating element and earlier such elements based on electrically conductive wires threaded through an electrically conductive body is that a small physical damage in the element, such as a hole, will shut off the electrical current and thereby the function of the element.
A further problem is that most known PTC materials comprise conductive particles such as carbon black in a crystalline polymer matrix. When the material is heated it expands and the resistivity increases as the gaps between conductive particles and between particle clusters increase. At approximately the polymer melting point a sharp rise in resistivity is obtained, the material “trips”, when the polymer softens and melts. This effect is due, not only to increasing distances between particles, but also to the movement of the particles and particle clusters in the melt and the breaking up of particle clusters obtained by the increased energy and movement of the particles within the clusters. On account of these considerable changes within the material, it shows a strong hysteresis effect, and hence the material will not return to its original properties after cooling. Further, as the tripping event is linked to the polymer melting point, it is difficult to adjust the level of the trip temperature.